Fuel cells produce electrical energy directly from an electrochemical oxygen reduction reaction from hydrogen as the fuel and oxygen as the oxidant. The operating principle is well known to those skilled in the art. Hydrogen gas (H2) brought into contact with an anode is converted into hydrogen protons (2H+) releasing two electrons (2e−), injected into an electric circuit. This is the hydrogen oxidation reaction. The hydrogen protons (2H+) then pass through a polymer membrane, or electrolyte, which separates them from a cathode. On coming into contact with the cathode, they are combined with oxygen gas (½O2) and two electrons (2e−), supplied by the electric circuit, to produce water (H2O). This is the oxygen reduction reaction. These electrochemical reactions generate an electric current, for operating an engine or motor, or charging a battery, and a reaction product, water.
FIG. 1 illustrates an exploded view of a fuel cell example that operates on the model described above. Fuel cell 1 is formed in a conventional manner of a basic cell 2 formed of two bipolar flow field plates 10, between which there is a membrane electrode assembly 11, whose structured is described below. The stack is then held between two end plates 20, 21, and forms a compact block that is held compressed by means of insulating pins 30, which extend from one end plate 20 to the other 21, and are fitted with nuts 40 at the ends thereof. Springs 50 mounted on the insulating pins and inserted between nuts 40 and end plates 20, 21, apply a compression force to the block parallel to the stacking direction. The hydrogen and oxygen gases are introduced and extracted from cell 1 using inlet and outlet connections that are not visible in plate 20. These connections are for connecting a gas supply circuit (not shown) and its control system, which is generally complex and voluminous.
Membrane electrode assemblies 11 (MEA), comprise a polymer membrane, which is electrically insulating but permeable to H+ ions. This membrane is arranged between an anode and a cathode formed of electrically conductive porous layers, at the core of which the electrochemical hydrogen oxidation and oxygen reduction reactions are produced. Said layers generally include a first gas diffusion sublayer and a second catalyst sublayer for the electrochemical reactions. Distribution channels 15 run over the faces of bipolar flow field plates 10 that come into contact with membrane electrode assemblies 11, ensuring that the gases are brought to and distributed on the active layers. The bipolar flow field plates 10 are formed of an electrically conductive material so as to collect the current generated by the hydrogen oxidation.
The operating principle of fuel cell 1 is simple but it is complex to implement. It is known, for example, that the ionic permeation of polymer membranes depends upon their water content. To operate properly, they must maintain a certain level of humidity at the core of the cell, by humidifying the injected gases. It is also known that the fuel cell reaction product is water. This water forms on the cathode side and then flows, as vapour, essentially in oxygen distribution channels 15. The presence of water in the fuel cell is thus both necessary and inevitable. However, it is problematic in that water vapour is liable to condensate and form drops that can, if they are numerous, block distribution channels 15 of one or more basic cells 2. The consequences, if the liquid water is not quickly removed, are a dramatic drop in the fuel cell's power and even destruction of the blocked cell. Monitoring the humidity level in fuel cell 1 and, more specifically liquid water formation, is consequently a significant factor in the operation thereof.
Some methods already exist for preventing water drops forming in the fuel cell distribution channels and optimising the level of humidity in the cell. One of these methods, disclosed in WO Patent Application No. 02006012953, essentially consists in checking the humidity level of the cell exhaust gases, then in removing a determined percentage of humid gas and re-injecting it at the cell inlet, after dehumidifying the remaining humid gas. This method is complex and expensive to apply. In order to be implemented, it requires a heavy and voluminous device. Moreover, it is not a method for specifically detecting liquid water formation in the cell distribution channels. Despite the checks carried out, it is not impossible for very localised temperature and pressure conditions to cause the formation of water droplets that are not immediately detected.